While a ship is moving through the water, it is desirable to know the speed at which the ship is moving. A variety of sensors for determining the ship's speed with respect to the water are presently available. These sensors include conventional paddle-wheel sensors, ultrasonic Doppler shift sensors, a pair of side by side acoustic transducers, or other devices.
The boundary layer at the hull of the ship creates difficulties in accurately measuring the speed because of friction between the hull and the water. A speed measurement within the boundary layer will be inaccurate. Therefore, most speed sensors must stick out of the hull beyond the boundary layer to provide an accurate measurement. This makes the sensor vulnerable to damage by debris in the water. It also creates considerable drag on the ship, which is a factor in sailboat racing and for high-speed motor boats.
Accurately measuring the depth of the water is also a frequent requirement on ships. A variety of depth measurement sensors are known in the art. A common depth sensor is an acoustic transducer which bounces an acoustic wave off of the bottom. The time lapse between the transmission and reception is proportional to the depth of the water.
There are many different versions of acoustic depth sensors, including flush mounted transducers, transom mounted transducers, faired bronze transducers extending from the hull, and the like. The advantage of the transom mount is that a hole through the hull is not necessary. The disadvantage is that extra drag on the ship is created and that the transducer is more vulnerable to being damaged. The advantage of a flush mount or mushroom mounted transducer is that less drag is created and that the transducer is more protected from being damaged. However, a hole must then be drilled through the hull for mounting the depth transducer, separate from any hole which was made for the speed transducer. It is therefore desirable to obtain a sensor which combines the functions of determining the speed of the ship as well as the depth of the water into a single housing that can be mounted flush with the hull of the ship.